Entropy Theory
by brumal
Summary: Although he was usually brimming with what Rin considered useless energy, next to Nagisa, who had even more useless energy, Aiichirou was getting quickly overpowered by Nagisa's strong personality.


**A/N:** If you would like to share this on Tumblr, please reblog it directly from my page! Just add "stories" to the end of my URL and you can find a masterlist of my works. Thanks!

Written for the prompts: "The guys take a liking to Nitori and treat him like a little brother, especially Nagisa because he's so used to having older siblings and Nitori is so timid he can't help but boss and drag him around, like a playmate! Rei, Makoto and Haru always remind him to cool it or he'll kill the poor kid but Nitori doesn't really mind. Rin, although he's glad the group likes Nitori, gets annoyed/jealous of the lack of Nitori's full attention," and "Gou and Nitori become friends and they go out and gossip about Rin all the time and Rin gets jealous both at the relationship Nitori has with his sister and how much time his sister is spending with Nitori. Eventually he tells Gou to back off (in a nice brotherly way) and accidentally lets it slip that Nitori is his boyfriend but Gou knew weeks before because Nitori can't keep secrets and Rin gets all pouty."

* * *

"Wait, wait, Ai-Chan! Come over here first!"

As usual, Nagisa's voice was loud and rambunctious. As he pulled on the silver-haired boy, he laughed and smiled the entire time. Rei was chasing after both of them, continuously scolding the blonde and telling him to take it easy. Intermittently, he included a hasty apology to Aiichirou for the rough-handling. Gou trotted after them, her scarf coming unraveled as she laughed and tried to keep Rei from stressing out too much and dying.

It was all the three second-years could do to watch as the four of them fooled around and chased each other in circles in the small park. They didn't say much as they walked, until Makoto laughed slightly and commented, "Nagisa is as energetic as ever, isn't he?"

Next to him, Haruka said nothing, but Rin snorted. "He's taking the poor kid on a joyride or something."

"It does look that way, doesn't it?" The tallest of them smiled. "But I guess it can't be helped. Nagisa always gets so excited with new friends."

To that, the redhead said nothing. His eyes followed the noisy first-years, his gaze settling a little linger on Aiichirou, who was laughing cheerfully as Nagisa hung on him.

It seemed that he was having a lot of fun, but, as usual, he was being politely accommodating and going with whatever fanciful whim Nagisa threw at him. Although he was usually brimming with what Rin considered useless energy, next to Nagisa, who had even more useless energy, Aiichirou was getting quickly overpowered by Nagisa's strong personality.

Rin shuffled through the autumn leaves which littered the edges of the sidewalk, his hands firmly pressed into his pockets. Even though Nagisa was borderline bullying the boy with enthusiasm, it was nice to see Aiichirou getting to interact with friends in his own grade. The silver-haired boy spent so much time hanging out with him that Rin worried he wasn't getting enough time with his other friends.

Now, with it being colder, the Iwatobi swim team had more joint practices with the Samezuka club. Consequently, their opportunities to hang out with all seven of them increased as well.

Today was the same as every other day after joint practice. Nagisa had suggested that they grab food together, and now they were heading back to their respective homes and campuses.

Autumn was sneaking in on them slyly, first lulling the summer flowers to sleep, then dulling all the foliage into a brown-green. But then it got loud again—and pleasantly so. Vibrant warm hues bloomed at the edges of tree branches, showing a shy blushing of reds and oranges and yellows against waning greens. Fallen leaves coated the ground to provide a crunchy carpet to step on between destinations.

This was the transitory beauty of autumn before winter's touch became too deep and stripped the trees of everything. Until then, it would only be romantic walks down quiet paths.

Now, however, the walk was anything but romantic. Nagisa was still crowing over Aiichirou, but with Rei tugging at his arm and holding him back, he was a little quieter. As the blonde somehow tricked the taller boy to spin around with him with their hands interlinked, Gou caught up with Aiichirou, laughing. She patted him on the arm lightly and started chatting with him.

Rin ignored Nagisa and Rei (who was very indignantly yelling things like, "Nagisa-kun, please stop that!" "My glasses are slipping off!" and "I-I'm getting too dizzy, Nagisa-kun...!") and focused on Aiichirou and younger sister.

Aiichirou seemed more relaxed, now that Nagisa was out of his hair. His smile was much softer, and he was attentively watching the girl as she spoke. For a moment, she linked arms with him good-humouredly and leaned into his ear to whisper something which sent both of them into a giggling fit.

The next second, Nagisa pulled over a very unsteady-looking Rei to them and engaging them in an unexpected and abrupt group hug. It was a miracle they didn't all fall and break something, probably Rei.

"Will that Nagisa ever grow up?" Rin asked exasperatedly. The blonde didn't seem to have changed at all since their days back in elementary school.

"Nagisa's Nagisa," Makoto said kindly. "You'd miss him if he changed."

Rin only gave him a noise of disinterest in return.

Haruka, who had been quiet the entire time but was watching the first-years with attentiveness, finally spoke up a little and called out to the group.

"Nagisa." His voice wasn't raised very much, but at the rarity of hearing it call him, Nagisa immediately perked up, let go of Rei, and happily trotted over.

Haruka and Makoto slowed down their pace as they talked with Nagisa, but Rin pressed on without change. He started to break apart from their small group. Rin guessed that Haruka was trying to give the silver-haired boy a break from the mood maker of the group.

Gou also looked back to them when Haruka called the blonde and tilted her head curiously. With a quick word excusing herself from Rei and Aiichirou, who were now engaged in what Rin could only assume was a furiously polite verbal battle between who had the most modest forms of speech, she went over to her brother.

"Onii-chan!" she called out as she neared him, her scarf rewrapped around her shoulders.

He paused for a second to let her catch up. "What?" he asked gruffly, peering at her out of the corners of his eyes.

She was completely unimpressed with his show of surliness and only smiled more. "You look so disgruntled," she said, "Like this." Gou furrowed her eyebrows furiously and stuck out her lower lip.

"Ha? I definitely am not pouting," he said. His eyebrows were furrowed together though.

The girl laughed and bumped her shoulder into him. "Don't be a spoilsport, onii-chan!" She looked at him curiously. "What's wrong?"

He gave her a suspicious leer. "Nothing's wrong. What're you going on about?"

"Onii-chan, you can't fool your favorite little sister! You definitely look like something's on your mind."

He resisted the need to roll his eyes at her cheesy line. "All of you guys are crowding Nitori to death," he said neutrally, taking his elbow away from his body so that his sister could tuck her hands onto his arm. Her small hands were warm against his side.

She giggled. "You're just overworrying like Rei-kun, onii-chan." The two siblings looked over to Rei and Aiichirou, whose energy levels seemed to be ramping up exponentially as their verb conjugations intensified and words became lengthier.

"Unbelievable," Rin muttered, shaking his head. Only those two would ever carry out an entire conversation in polite form.

Gou laughed at him. "Aiichirou-kun sure is enjoying himself."

Her brother's reply to her was a soft, back-of-the-throat noise. "It's good for him to hang out with other first-years," he said after a begrudging moment.

She hummed lightly into his arm. Her cheek rested comfortably in the folds of his jacket.

"Hey, stop stealing my warmth," he scolded, nudging her face gently. He let a small smile edge itself onto his lips when all she did was nuzzle even closer into him. After a beat, he asked, "And what's with you sticking to him all the time anyway?"

"Onii-chan, are you jealous?" she asked, her eyes wide in mock-disbelief.

He shook his arm a little, as if trying to get rid of her annoying question. "What're you talking about?"

"I dunno," she said playfully, pushing his arm back and forth like she did when they were much younger. "For a second, you sounded like you were really jealous."

Her brother snorted, but the sour look on his face remained.

Gou shivered a little when a particularly nippy breeze caught her off-guard. Behind them, Makoto was dealing with an enthusiastic Nagisa, who was trying to cop a piggy-back ride out of him.

"If you're not jealous, then it should be fine for me to hang out with Aiichirou-kun, right? What's the problem with that?"

The redheaded boy became huffy and frowned. "The problem is that you're my little sister and he's my boyfri—"

His voice cut off, and all that was left was the background din of Nagisa's voice mingling with the crunching of the leaves they were stepping on.

It could have been very possible that Gou didn't hear her brother's slip-up. She was looking at him, since he had fallen quiet so suddenly. Rin was flushing red and had his face shoved deep into the folds of his muffler.

"Onii-chan?" Gou asked. When he didn't respond, she nudged him. "Hey, onii-chan, what's wrong?"

"... You didn't hear anything, did you?"

"Hm? You mean the part where I'm your little sister? Well, of course, right? I heard that!"

Rin tensed for a second and waited for her to continue. When she didn't but just kept walking quietly, he relaxed.

"But, you know, I also heard that part about you and Aiichirou-kun being boyfr—"

At that, Rin immediately took his hand out of his pocket and pressed his fingers against her mouth. She didn't even flinch at his furious glare, having grown used to it after growing up with him. After assessing the mischievous sparkle in her eyes, Rin reached the conclusion that she had been hanging out with Nagisa too much as well.

"I thought—" he started in a furious undertone but failed to finish. "You said—What the hell, Gou!"

She tapped at his fingers with her own hands, all the while smiling mirthfully with her eyes. Rin was not pleased at how wide her grin had gotten underneath his palm. Regardless, he relented and let go of her.

"I knew you'd let it slip eventually!" she exclaimed happily. She giggled gleefully. Her slender fingers were wrapped around his as she breathed the words excitedly into his palm.

"Eventu—What do you mean 'eventually'?" he whispered heatedly, his eyes glancing between the others to make sure no one was listening in on their conversation.

"Onii-chan, I've known that you and Aiichirou-kun have been dating _weeks_ ago!" Gou replied, patting her brother on the hand soothingly. She helped him shove it back into his pocket and encouraged him to keep walking, the warm smile on her face never leaving.

Rin didn't budge from his spot, however, making Gou halt as well. "Did he tell you?" he asked, eyes flashing dangerously.

Gou's eyebrows rose curiously. "Of course. Aiichirou-kun and I are friends, after all. I asked him about you two, and he told me after practice one day." She grinned even more widely. "I was just waiting for _you_ to admit to it, since I know you're so shy about these things."

"I'm not—I'm not _shy_ about 'these things'!" the boy said, blush betraying the falsity of his words.

"Don't worry, onii-chan! Your secret's safe with me," she said reassuringly, once again leaning her head against his shoulder. This time, Rin allowed her to pull him into a reluctant walk. "Although," she started again, "I'm not so sure about how safe it would be if Nagisa-kun caught wind of it."

Rin groaned audibly. Knowing the blonde, it wasn't so much a matter of "if" Nagisa caught wind of it. It was just an issue of "when." Then again, Rin really wouldn't be surprised if the blonde had managed to blackmail and extort the news out of Aiichirou already. The silver-haired boy was so weak against strong personalities, and Nagisa had one of the strongest personalities Rin was aware of. He rubbed his forehead with his free hand.

Behind them, Nagisa had apparently succeeded in getting Makoto to carry him, because he was laughing his head off like a maniac while the taller boy tried to keep them both upright. Rei and Aiichirou had turned to look at them too, on account for how loud Nagisa had gotten. Rei paled and excused himself from the silver-haired boy to get the blonde under control again.

Aiichirou smiled at the scene for a second before he caught Rin's eye and the curve on his lips softened. Gou noticed this too, patted her brother on the arm, and ran off to harass the Iwatobi boys. At that, Rin embarrassedly looked away from the boy and rubbed the back of his head. Without any prompting, Aiichirou began to walk over to the redhead.

"Rin-senpai," he said simply, the gentle smile on his lips still in place.

Rin grunted at him but didn't say anything. He didn't spare the others a glance as he started to walk slowly. As he always did, Aiichirou followed him and stayed by his side.

"Nagisa-kun and the others sure are lively today," Aiichirou commented happily, his cheeks and nose red from the cold and running around he had been involved in earlier.

"You doin' alright?" the redhead asked gruffly, his feet still shuffling in the fallen leaves.

The silver-haired boy nodded firmly. "Yes! It's wonderful being with everyone." Aiichirou watched as Rin stepped on the colorful leaves on the edge of the street and started to do the same, a small giggle coming from his chest.

"I thought Nagisa was going to kill you with his energy or something."

A wind tousled their hair, making Aiichirou shake his head to get rid of the cold. "Mmn, not at all. Well, he's a little too enthusiastic, but he means well."

Other than a tiny, tongue-clicking noise, Rin didn't give Aiichirou any other sign of acknowledgment.

"No, wait, Nagisa-kun—stop! I'm not going to carry you, so stop climbing over me already—Nagisa-_kun_!"

"Whee! Rei-chan, stop _moving_ already! Hehehe! C'mon, c'mon, just hang onto my legs for a second—"

The ruckus behind them rose up again as Nagisa decided to bother Rei instead. Makoto was trying to keep them from tripping over something, and Gou was hiding behind Haruka in case something terrible happened. The Iwatobi swim team was a mess.

Once the noise settled down a little, Rin groused, "You told Gou."

"Huh?"

"You told Gou," he repeated, voice low, "about…." Rin trailed off and frowned.

It took a second for Aiichirou to catch the redhead's drift. "Oh!" He blushed a little. "Oh, um, G-Gou-san asked so politely that she caught me off guard and I just…. Sorry, Senpai. I didn't mean to…."

Rin let out a long-suffering sigh. "Well, whatever. I guess it can't be helped anymore." He eyed Aiichirou suspiciously. "What about Nagisa?"

"Ah, um…." Aiichirou furrowed his eyebrows apologetically. "Nagisa-kun was much more persistent about it, so I couldn't resist…."

"I knew it," he muttered.

"S-Sorry, Rin-senpai…" the silver-haired boy apologized again.

Rin breathed in deeply and let it out again. "S'alright. Not like we can do anything about it anyway." He glanced over at the shorter boy and reached up to his hair. Aiichirou stilled when he noticed that Rin's hand was getting closer to him. "At least the little punk isn't doing anything to blackmail us or something." When the redhead retracted his hand, he held a small leaf which had gotten stuck in Aiichirou's hair. He tossed it away and shoved his hands back into his pocket.

The shorter boy smiled. "I think they're just happy for us."

Quietly, the redhead hummed.

"I'm, um…" Aiichirou looked down at his hands, which were wrung together, "I'm happy too."

"What for?" Rin asked.

"F-For us. I'm happy… for us," the silver-haired boy said hesitantly, his head completely ducked down. His ears were tinted red, and Rin knew it wasn't just from the cold.

He let himself smile and took his hand out of his pocket. Lightly, he rested it on Aiichirou's head and ruffled his hair.

"Yeah," he said simply. "I'm happy for us too."


End file.
